The present invention relates to a tensioner for the chain or the belt, for example for the camshaft drive of an internal combustion engine, and in particular to a tensioner of a type including a cylindrical tensioner housing with a piston loaded by a compression spring arranged therein, the tensioner housing and the piston being axially displaceable relative to each other, and the element which is displaceable relative to the fixed element in chain tensioning direction being surrounded by a first stop ring which limits the backward movement thereof.
Such a tensioner is arranged in the slack side of the chain drive and the compression spring transmits the tensioning forces into the slack side of the control gear. The mean tensioning forces occurring here during normal operation are lower than in the working or tensioned side so that the compression spring can be configured in accordance with these lower tensioning forces. The piston and tensioner housing assembly acting on the chain extends as a result of the force of the compression spring and thus produces the required chain tensioning force.
This assembly can be configured as a hydraulic damping device with an oil reservoir, a high pressure chamber and an intermediate non-return valve. When the piston and tensioner housing assembly is loaded by force pulses from the chain drive, hydraulic oil contained therein, which, due to its pressure, assists the action of the compression spring, can be pressed out of the high pressure chamber through the leak. gap formed between the piston and the tensioner housing by overcoming the fluid friction.
In special cases, when the high pressure chamber is not yet, or only partly filled with oil and the oil pressure in the high pressure chamber has not yet, or only partly been built-up, the required chain tension cannot be established by such a tensioner. Force pulses from the chain drive can then lead to a sudden retraction of the tensioner because the damping effect is inadequate. One example of such a special case for a vehicle engine is when the vehicle is parked uphill in driving direction with a forward gear engaged so that the engine tends to rotate in reverse direction due to the weight of the vehicle. This produces tension in the slack side of the chain. In such a case, if no stop ring is provided for limiting backward movement, the high chain tension leads to the piston and tensioner housing assembly of the hydraulic pressure generator being completely retracted after a longer standstill of the engine. If the engine in now started causing a sudden relaxation of the slack side, the chain tension is insufficient because the piston is greatly retracted so that uncontrolled knocking of the chain can result. This can lead to the chain jumping one or more teeth of the drive pinion of the camshaft drive. This causes a misadjustment of timing in the valve gear which results in damage to the engine, for example due to the valve disks striking against the piston bottoms.
A tensioner of the initially cited type is known from DE-OS 36 36 919. In this tensioner, the piston which can extend out of the fixed tensioner housing acts in tensioning direction on the chain to be tensioned. When this chain tensioner is set into operation, the first stop ring snaps into a first locking groove of the tensioner housing. This position is the beginning of the chain tensioning range. When the pressure in the high pressure chamber is not sufficient to bring the piston to a stop under load, a stop edge of the piston comes to abut against the first stop ring. This stops the backward movement of the piston into the tensioner housing. The stop ring therefore acts as a safety device against a back stroke or sinking.
Stop rings of this type are made of wire which must have a certain minimum wire diameter because of the combined effect of the dimensional tolerances and strength properties of the components. With this minimum diameter, the wire of the stop ring whose ring diameter has to be slightly adjustable during operation, already possesses a high degree of stiffness which increases considerably when the ring diameter diminishes and the wire diameter remains the same.